Ruddlesden-Popper halide perovskites are 2D solution-processed quantum wells with a general formula A2A’n-1MnX3n+1, where A, A’ are cations, M is a metal, X is a halide, and their physical properties can be tuned by varying the perovskite layer thickness (n value). They have recently emerged as efficient semiconductors for optoelectronics [1-3]. However, fundamental questions concerning the nature of optical resonances, their scaling with quantum well thickness, and the physics behind the exciton properties, remain unresolved. Here, using optical spectroscopy and 60-Tesla magneto-absorption supported by modelling, we unambiguously demonstrate that the optical resonances arise from tightly bound excitons with binding energies varying from 470 meV to 125 meV with increasing thickness from n=1 to 5 [4]. Comprehensive modelling of exciton states enable the understanding of dielectric confinement effects which prevail over quantum confinement in 2D perovskites. From these results we produce a general scaling behaviour for the binding energy of exciton states in Ruddlesden-Popper perovskites. [1] Tsai et al., Nature (2016), 536, 312-316. [2] M. Yuan et al., Nat. Nanotechnol. (2016), 11, 872-877. [3] Blancon et al., Science (2017), 355, 1288-1292. [4] Blancon et al., arXiv:1710.07653.